// Copyright 2014 Google Inc.  All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package ansi

import (
	"bytes"
	"errors"
	"strings"
)

// An S contains a decoded escape sequence.  There are several types of
// escape sequences:
//
//   "C0"  - Single Control Set 0 character 0x00 - 0x1f
//   "C1"  - Single Control Set 1 character ESC 0x40 - 0x5f / 0x80 - 0x9f
//   "ICF" - Independent Control Function ESC 0x6f - 0x7e
//   "ESC" - Other escape sequence
//   "CSI" - Standard ANSI Escape Sequence ESC [ ...
//   "CS"  - Control String
//   ""    - String of regular text (no ESC or C1 characters)
//
// CSI escape sequences may contain some number of parameters.  The parsed
// parameters are provided in Parmas.  Control Strings, such as OSC, have the
// string stored in Params[0].  Some C1 sequences take additional bytes of
// data following the sequence, such as SS2 and SS3.  In this case the
// additional data is in Params[0].
type S struct {
	Code   Name     // The escape sequences sans parameters
	Type   string   // The type of escape sequence
	Params []string // parameters
}

const (
	sos = (1 << iota) // start of string
	st                // string terminator
)

// lookup provides a quick lookup of byte attributes
var lookup = [256]byte{
	'_':  sos, // APC
	'P':  sos, // DCS
	']':  sos, // OSC
	'^':  sos, // PM
	'X':  sos, // SOS
	'\\': st,  // ST
}

// Fixed error messages generated by package ansi.
var (
	LoneEscape        = errors.New("escape at end of input")
	UnknownEscape     = errors.New("unknown escape sequence")
	NoST              = errors.New("control string missing string terminator")
	FoundSOS          = errors.New("start of string encountered in control string")
	IncompleteCSI     = errors.New("incomplete control sequence")
	ExtraParameters   = errors.New("too many parameters for function")
	MissingParameters = errors.New("not enough parameters for function")
)

// ms returns the bytes of in as a Name
func ms(in ...byte) Name { return Name(in) }

// Decode decodes the next sequence in in, returning the bytes following
// the sequence.  The sequence S, and any possible error.  Single byte C1
// sequences are expanded to two byte sequences
func Decode(in []byte) (out []byte, s *S, err error) {
	if len(in) == 0 {
		return nil, nil, nil
	}

	// TODO(borman): should we split out C0 characters?

	// If the first byte is not an ESC or C1 code then return everything
	// up to the first ESC or C1 code.
	for x, c := range in {
		if c == '\033' || (c&0xe0 == 0x80) {
			if x > 0 {
				return in[x:], &S{Code: Name(in[:x])}, nil
			}
			goto EscapeSequence
		}
	}

	// If we get here the entire string has no escape sequences.
	return nil, &S{Code: Name(in)}, nil

EscapeSequence:

	// Some codes require additional post-processing.  For example,
	// SS2 and SS3 also gobble up the next character, which we
	// return as a parameter.
	defer func() {
		if s == nil {
			return
		}
		if f := Specials[s.Code]; f != nil {
			out = f(out, s)
		}
	}()

	// Any parameters (or control string) found is stored in params.
	// Upon exit we convert it to a slice of parameters.  In the case
	// of a control string there will be at most one parameter.
	// If params is nil then there are no parameters.  If params is
	// empty then there is exactly one empty parameter.
	var params []byte
	defer func() {
		if s == nil {
			return
		}
		if params != nil {
			// 5.4.1: The Parameter Bytes are bit combinations from
			// 03/00 to 03/15. The parameter string is interpreted
			// as follows:
			//
			// a) If the first bit combination of the parameter
			// string is in the range 03/00 to 03/11, the parameter
			// string is interpreted according to the format
			// described in 5.4.2.
			//
			// b) If the first bit combination of the parameter
			// string is in the range 03/12 to 03/15, the parameter
			// string is available for private (or experimental)
			// use. Its format and meaning are not defined by this
			// Standard.
			//
			// 5.4.1:
			//
			// b) Each parameter sub-string consists of one or more
			// bit combinations from 03/00 to 03/10; the bit
			// combinations from 03/00 to 03/09 represent the digits
			// ZERO to NINE; bit combination 03/10 may be used as a
			// separator in a parameter sub-string, for example, to
			// separate the fractional part of a decimal number from
			// the integer part of that number.
			//
			// c) Parameter sub-strings are separated by one bit
			// combination 03/11.
			if len(params) > 0 && s.Type != "CS" && params[0] <= 0x3b {
				s.Params = strings.Split(string(params), ";")
			} else {
				s.Params = []string{string(params)}
			}
		}
		// Only CSI sequences require additional processing
		// If our code is CSI then we don't know how to validate
		// the paramters (and there are no defaults).
		if s.Type != "CSI" || s.Code == CSI {
			return
		}

		// Now fill in default parameters
		c := Table[s.Code]
		if c == nil {
			return
		}

		// Validate the number of parameters we were given.
		switch {
		case c.NParam == 0 && len(s.Params) > 0:
			err = ExtraParameters
		case len(s.Params) < c.MinParam:
			err = MissingParameters
		case c.NParam == -1:
		case len(s.Params) > c.NParam:
			err = ExtraParameters
		}

		for x, d := range c.Defaults {
			if len(s.Params) == x {
				s.Params = append(s.Params, d)
			} else if s.Params[x] == "" {
				s.Params[x] = d
			}
		}
		// Add in empty missing parameters
		for len(s.Params) < c.MinParam {
			s.Params = append(s.Params, "")
		}
	}()

	switch {
	// Single byte CSI
	case in[0] == '['+0x40:
		in = in[1:]

	// Single byte C1 (not an escape)
	case in[0] != '\033':
		return in[1:], &S{Code: ms('\033', in[0]-0x40), Type: "C1"}, nil

	// Lone escape
	case len(in) == 1:
		return in[1:], &S{Code: ms('\033'), Type: "C0"}, LoneEscape

	// Two byte CSI
	case in[1] == '[':
		in = in[2:]

	// Start of control string
	// Should terminate with ST
	// Cannot contain SOS
	case (lookup[in[1]] & sos) == sos:
		t := bytes.Index(in[2:], []byte{'\033', '\\'})
		var err error
		if t < 0 {
			t = len(in) - 2
			err = NoST
		}
		d := in[2 : 2+t]
		if i := bytes.Index(d, []byte{'\033', 'X'}); i >= 0 {
			d = d[:i]
			t = i
			err = FoundSOS
		}
		params = d
		// If ere not returning NoST then we need to consume
		// the two bytes that terminated us
		// Question: should we consume an SOS?
		if err != NoST {
			t += 2
		}
		return in[2+t:], &S{Code: ms(in[:2]...), Type: "CS"}, err

	// Two byte C1 string
	case in[1] >= 0x40 && in[1] <= 0x5f:
		return in[2:], &S{Code: ms(in[:2]...), Type: "C1"}, nil

	// Two byte independent control function
	case in[1] >= 0x60 && in[1] <= 0x7e:
		return in[2:], &S{Code: ms(in[:2]...), Type: "ICF"}, nil

	// Unkonwn escape sequence type
	default:
		return in[2:], &S{Code: ms(in[:2]...), Type: "ESC"}, UnknownEscape
	}

	// a CSI sequence has been encountered.  The sequence consists of
	// 0 or more parameters bytes (0x30 - 0x3f) followed by
	// 0 or more intermediate bytes (0x20 - 0x2f) followed by
	// 1 final byte (0x40 - 0x7e)

	s = &S{Type: "CSI", Code: CSI}

	// collect up all the parameterb bytes into params
	for x, c := range in {
		if c < 0x30 || c > 0x3f {
			if x > 0 {
				params = in[:x]
			}
			in = in[x:]
			goto Next
		}
	}

	// this sequence prematurely terminated (no intermediate or final bytes)
	if len(in) > 0 {
		params = in
	}
	return nil, s, IncompleteCSI
Next:
	// collect up all the intermediate bytes (0x20 - 0x2f)
	for x, c := range in {
		// Normal escape sequence
		if c >= 0x40 && c <= 0x7e {
			s.Code += Name(in[:x+1])
			return in[x+1:], s, nil
		}
		if c < 0x20 || c > 0x2f {
			s.Code += Name(in[:x+1])
			return in[x+1:], s, UnknownEscape
		}
	}

	// There was nothing following the intermediate bytes
	s.Code += Name(in)
	return nil, s, IncompleteCSI
}
